In supplying and controlling alternating electrical power to an essentially resistance-inductive load such as a large loop, as employed in electromagnetic geophysical exploration equipment, a difficult problem in large current arcing has been encountered. This arcing poses dangers to the operators and others near such equipment and leads to frequent wear and damage to the equipment. This problem exists in generating an alternating current and is especially severe in shutdown of the equipment. In the past this problem has been dealt with by simply replacing arc-worn contacts or equipment periodically and by isolating the human operator(s) from the equipment.